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Tuesday, January 27, 2015

Gear Coupling Tutorial - Part II: Configurations

A. Hubs and Sleeves

Flanged Sleeve Gear Coupling

Gear
couplings are made up of hubs which attach to the machinery shafts, and
sleeves that span the gap from one hub to the next. Sometimes the
sleeve is one piece as in the Sier-Bath & HercuFlex continuous
sleeve couplings and sometimes each hub has its own sleeve which in turn
bolts to the other half or other side of the coupling. The gear teeth
are found on both the hub and the sleeve of the flexible unit. The rigid
or non flexing piece could be a flange without teeth as in the Sier-Bath "F" & new HercuFlex "FX" type or could be a hub with straight teeth that acts like the fit of a spline shaft to a spline hub as in the Sier-Bath "C" & new HercuFlex "CX" type.

Continuous Sleeve Gear Coupling

The
flexible half coupling consists of a flexible hub and a sleeve. The hub
that attaches to the shaft is also the part with the crowned teeth. The
crowning includes tip crowns, flank crowns, and chamfers on the sharp
edges. Crowning helps improve tooth life as well as misalignment
capability. By crowning the teeth we improve the contact area from
tooth to tooth and reduce the pressure of the torque forces. It also
prevents the sharp edges of the tooth from digging in and locking the
coupling. Sleeve teeth are straight except for a chamfer on the minor
diameter edge.The gear teeth are narrower than the gap between
the teeth. That space is called the backlash or if you will the
looseness of the fit. Gear couplings always have some backlash. In
addition to contributing to the misalignment capabilities, the backlash
also provides space for the lubricant. Lubrication is necessary for gear
couplings to work well. Some gear couplings have more backlash than
others. In addition to the backlash there is another matching fit on the
hub to sleeve interface. That is the "major diameter fit". Gear
couplings are made to fit closely at the major hub diameter and the root
diameter of the sleeve. When the coupling is not rotating, those two
surfaces rest upon each other if it is a horizontal installation. In
operation the teeth mate at the pitch line, and that is where torque is
transmitted. Minor diameter fits would preclude suitable misalignment
capability and torque transmission capability.

Rigid hubs are
basically a flange, fitted to the shaft and bolted to the adjacent ½
coupling. Rigid "C" and "CX" couplings utilize one hub with straight
teeth that mate with the continuous sleeve. The straight tooth rigid
hub fits very tightly into the sleeve because there is very little
backlash. We do not want the rigid hub to flop around as it could cause
vibration problems.

The hub and sleeve of the gear coupling are
also fitted together so as to prevent the lubricant from leaking out.
Most gear couplings are lubricated with grease. When oil lubrication is
used it is usually a continuous flow through the tooth mesh. Oil
lubrication is a special case. The sleeve to hub interface at the
boundaries can have elastomer rings, gaskets, or labyrinths to prevent
grease leakage.

The two halves of a flanged type gear coupling
are bolted together. The bolting is an important part of the power
transmission path. Some designs have the power transmitted across the
face by friction in which case the bolt provides enough clamping force
to provide face friction. Other designs allow the bolts to carry the
load in shear. Both cases require a proper analysis of the multiple
loads on the bolts. In addition the bolt bodies may provide the
centering action to pilot the two halves of the coupling. Bolts can
either be exposed or shrouded. Sier-Bath originally promoted the
shrouded bolt as standard for safety reasons. With the advent of OSHA
and required coupling guards, some of the reason for the standard goes
away. The two types also have differing windage loss that affects the
installation. Bolts are not the common hardware store standard. (Please
do not try to use them!)

The
continuous sleeve or "C" and "CX" type of gear coupling is not bolted
together. That is an advantage in that the coupling can be made smaller
and lighter. Smaller, lighter couplings also have lower inertia values.
Lower inertia is an advantage when starting the machinery. Another
advantage of no bolts is no bolt stress. The bolts can be the weak point
in some applications. Under some conditions of balance and high speed
the bolts are also detrimental.

Lovejoy's
Sier-Bath and HercuFlex flanged sleeve ("F" and "FX") gear couplings
are built to American Gear Manufacturers Association (AGMA) dimensions
up to size 9. That means a Sier-Bath or HercuFlex flanged series
coupling will mate half for half with all other gear couplings built to
AGMA standards. While AGMA standards are US based, many European
manufacturers build to match the dimensions. Matching dimensions
include the interface only, such as outside flange diameter, number of
bolts, bolt size, bolt circle and gap. Many times the length through
bore of the hub is identical. No promise is made about torque or bore
capability. Lovejoy like other gear coupling manufacturers use similar
nomenclature to identify the coupling sizes. Each company publishes an
interchangeability chart up to size 7 at least.

B. Planes of Flexibility

We
could look at the plane of flexibility as a pivot point on the
connection of shaft to shaft. An elastomer coupling has one flex plane
where the elastomer distorts to provide the angular and parallel
capability. A full flex gear coupling has two pivot points at the two
gear meshes. A disc coupling has two sets of flexible metallic elements
that distort to provide the misalignment. Except for elastomer couplings
that distort in two directions, a flex plane only provides for angular
flexing. In a gear coupling that angular flexing is generally 1 1/2°.
Special design spindle couplings have more capability. For other types
of couplings you should refer to the literature as no two coupling types
are alike.

1. Radial or Parallel Misalignment

Two
flex planes, each providing some angular flex, are placed in series to
obtain parallel misalignment. The greater the axial distance from one
flex plane to the next, the greater is the radial or parallel
capability. Spindle couplings, and floating shaft couplings provide the
maximum capability. Spacer couplings are also good for extra radial
displacement and close coupled couplings such as a standard flanged
sleeve gear coupling provides the minimum.

2. Angular Misalignment

Flex-rigid
gear couplings provide only angular misalignment. There is only one
flex plane. No radial capabilities are provided by the flex rigid
coupling. The teeth can tilt within the mesh, but that is all the
displacement allowed although backlash could allow for some radial
misalignment too. Single element disc couplings also provide for angular
misalignment only. Single element elastomeric couplings may distort
enough to displace both radial as well as angular misalignment.3. Axial Misalignment

Axial
misalignment is also handled by the gear coupling. Axial displacement
is available from either the full flex or the flex rigid unit. The
amount available depends on many factors, and in fact specials are
available for long sliding applications. Axial misalignment or movement
is often associated with thermal shaft growth and floating rotors. It is
accommodated by the sliding action of hub tooth within sleeve tooth.
The axial movement can be stopped with a plate or a button. Other types
have little or no capability in that plane of misalignment; some
elastomeric couplings even have difficulty stopping the axial f1oat
after a specific distance and may fall apart.

There are many
combinations of angles and spacing which can be calculated by plane
geometry, to obtain the ideal situation for an application. Always keep
in mind that equipment should be aligned to the rotating equipment
manufacturers' standards and requirements, not only the coupling
manufacturers. When operating misaligned, the coupling could transmit
reactionary loads or vibrations that are within the coupling
capabilities, but not the equipment capabilities.

C. Hardware and Accessories

Nuts
and bolts, grease seals, back up rings and gaskets are all needed for
the gear coupling. Except for the bolts, these items are used for
holding grease in the coupling. Sometimes these items limit the coupling
application. For example, the temperature may be limited to the o-ring
capabilities. Misalignment may allow grease to leak out the seal
surface, or some modifications may need a wiper seal rather than an
o-ring. The seals can be held in place by several means. The o-ring is
the most simple, and it fits into a groove in the sleeve. Sometimes the
seal holder is bolted to the coupling sleeve. This is always the case
on couplings larger than size 9. It makes the assembly of the coupling
to the shaft easier, and makes replacement of seals easier. These
couplings with bolt on seal carriers are designated FHD. "F" series
couplings 7 through 9 can be either the "HD" version or the plain
version.

The "FA" type of Sier-Bath & "FX" HercuFlex
couplings uses a high misalignment seal with more flex than the regular
seal. The "C" coupling seal is held in place by a "spirol" ring and it
has stiffeners molded into the inside face. It is a U or C shape that
stays closed under load. It also provides the movement limit for the
coupling and is actually rated to withstand an end force.
Grease
of course is an important gear coupling accessory. Coupling grease is
not ordinary grease but is specially formulated so the oils do not
separate from the soaps. The result is that the lubricant is contained
within the needed space and sludge is not allowed to accumulate. Oil and
soaps separate in ordinary lubricants because of centrifugal forces on
the heavier particles.

D. Variations to Standard Couplings

1. Fill the Space Between Shafts

Couplings
often must fill a space between shafts as one of their primary
attributes. It would seem a simple enough task, but not all couplings
offer flexibility doing that job. This is another reason why the gear
coupling is very popular. The distance between shaft ends of rotating
machinery must vary to accommodate design standards, product line
variations, different motor frames and maintenance needs. A gear
coupling can meet those needs in a variety of ways.

If
the gap between shafts is small the coupling can utilize its capability
to reverse the hubs or face off the hubs. An infinite number of
possibilities is can be obtained from catalog minimum to catalog
maximum. Note this gap does not affect the distance between flex planes
unless the hubs are reversed. One or both hubs can be reversed.
Sometimes a spacer piece is used to allow for maintenance space or
machine removal.

Spacers
are built to standards for the machinery builders. Pumps have several
standard spacers such as 3 1/2 inches, 7 inches and others. Compressors
would have a different set. Spacers use two ½ couplings and one flanged
hollow tube to connect the coupling halves. Spacers can serve to
separate the flex planes and can be part of the torsional tuning of a
coupling.

Spacers
have practical limits on length that are associated with cost, weight
and critical speed. When the spacer becomes too costly, the next step is
to use a floating shaft coupling to achieve the necessary spacing. The
primary reason for a long floating shaft is to achieve greater radial
misalignment between shafts. The secondary reason is to reach a long
distance between the driver and the rotating equipment. The floating
shaft coupling consists of two flex rigid couplings connected by a piece
of solid shafting. Floating shafts are found on bridge cranes and steel
rolling mills. Weight and critical speed are important considerations
for floating shafts.

Gear couplings can be modified to allow
shaft growth in the axial direction and to limit growth in the axial
direction. Limiting the growth calls for a plate and possibly a button
to be inserted between the coupling halves. As the shaft tries to move
in the axial direction it is stopped after moving a predetermined
distance. That is what we mean by "limited end float". It is necessary
with sleeve bearing motors. Sleeve bearings are often used in the large
motors over 200 horsepower. Those same plates and buttons are used on
vertical couplings.

Sometimes
it is not movement that we want to stop, but the electrical current
known as galvanic current. To do that we offer an insulated coupling.
One half of the coupling is electrically insulated from the other
thereby interrupting the flow of current. It is done by adding
insulating plates and bushings. Galvanic currents are the cause of
pitting and corrosion at close running fits of mechanical equipment and
of the gear teeth. It is not necessarily a high voltage insulator as
found in wiring systems.

On the other hand, the gear coupling can
be arranged to allow axial float, as would be required by thermal
growth of a shaft in a hot application. Many couplings can be set to
allow some thermal growth, but only the gear coupling freely slides back
and forth without adding to the load on the flex element. In addition
to thermal growth, gear couplings can be arranged to slide great
distances. The long sliders are used for removing equipment from the
system where the coupling is the most suitable point of movement. Medium
sliders are used when the coupling must adjust axially while the
machinery is in motion doing its job. Refiners, Jordan machines, and
roll winders found in paper mills utilize the sliding capability.
Spindle couplings also have some slide capability to adjust to the
installation or operational requirements of rolling mills.

2. Modify the Coupling to Satisfy Some Customer Needs

Some
applications extend beyond the traditional coupling requirements of
torque transfer, misalignment, and filling space. A simple case is the
vertical coupling. When the coupling is mounted vertically, the forces
and weights need to be accounted for, since, we cannot allow them to
interfere with the misalignment action. For gear couplings we must
account for the sleeve weight and potential movement. That is
accomplished by adding a plate and button as mentioned before under the
limited end float discussion. The button is rounded to allow the load to
transfer under misalignment. The load or weight is transferred to the
lower shaft and thence to a thrust bearing in that equipment. Hanging
load couplings in turn transfer the load upward to a bearing in the
upper coupling. In a vertical floating shaft coupling the entire
floating assembly rests on the lower shaft and must be accounted for by
the designer.

Gear couplings can be configured to do special
jobs. We already discussed the slider possibilities, other possibilities
include the shear pin, cutout, and brake coupling. Shear pin couplings
disconnect when subjected to predetermined torque overloads thus
protecting other equipment. Torque overloads could come from stalls or
cyclic overloads. Cut-out couplings, which can be automatic or manual
and include pins to hold them in position, function as a disconnect or a
connect coupling. They could be used on a dual drive machine to isolate
the unused driver. They could be used for a temporary connection for
adjustment of a rotary shaft, in which case they connect and disconnect
on the fly. They could also be used for a turning gear that rotates
heavy equipment when it is off line, and helps prevent a permanent set
in the shaft. The cutout pin holds the coupling in one position or the
other. A brake wheel or brake disc coupling includes a brake device on
the coupling. It is a matter of space conservation in some systems to
put the brake on the coupling. In other situations putting the brake at
the coupling prevents the high cyclic torque from reaching low torque
shafts. Brake wheel couplings are often attached near the gear box shaft
since the gear inertia is in the box.

E. Moderate and High Speed Applications

At
the beginning of our discussion of gear couplings we mentioned that
gear couplings are capable of very high speeds. They can do speed and
torque at the same time. The limit has always been the need for
lubrication of the mating gear surfaces. While high speeds increase the
wear rate and can be the cause of high stresses within the coupling, the
big issue is balance. Couplings operating at high RPM or high rim speed
will cause vibration problems if they are not in balance. Balance and
radial deflection also plays a big role in the issue of lateral critical
speeds.

Coupling balance is achieved through design,
manufacturing and balancing machines. Without going through a complete
dialog of motion mechanics, let us say that balance concerns itself with
how the weight of the rotating mass or inertia is positioned or
displaced relative to the center of mass. If that weight is perfectly
distributed around the center of rotation and the center of the
coupling, the coupling is in balance. Since nothing is perfect in
couplings and some other issues in life, there is always a potential
unbalance. Some of the potential unbalance is a result of machining
tolerance. Off center, out of round, non-parallel or even loose fits
lead to mass displacement. In castings some of the potential unbalance
could come from voids or air space internal to the casting. When a
coupling consists of an assembly, the design and the assembly process
can result in an unbalance condition. To have the best balance it is
necessary to be balance proactive in the design and tolerances, and to
have very tight tolerances. The final step then becomes one of putting
the coupling or its components on a balance machine to measure the
unbalance and to take some countermeasures. Countermeasures usual
involve removing material in certain areas to compensate for the mass
displacement. Balance is also accomplished in some cases by addition of
material. In the end there will always be some residual unbalance.

Residual unbalance specifications and balance methods are the subject of several standards.The
standards apply to all types of couplings even low speed units in some
cases. Specifications have been developed by AGMA, ISO, and API to name
three. Individual OEM customers may have their own standards that are
usually taken from the recognized standards listed previously. Imperial
unit unbalance is measured in inches (usually micro inches), or in
oz-inches. In the former case the dimension locates the center of mass
relative to the rotation, while in the latter case the unbalance is
identified in both weight and distance. Many specifications base the
standard on operational speed as well.

Gear couplings present a
special case for balance problems. A gear coupling has a backlash and
loose fit between the teeth so it cannot be spun on a balance machine
while assembled without modification. The gear coupling is assembled
with a slight interference on the major diameter, balanced,
disassembled, ground at the tooth tips and reassembled for use in the
application. Gear couplings will not work without the looseness at the
major diameter.

While balance is very important to high speed
gear couplings, it must also be noted that high speed has the potential
for high wear of the teeth. Extremely high speed units utilize hardened
teeth to extend the coupling life. Slider couplings have hardened teeth
in some applications too. In a misaligned gear coupling the hub teeth
and sleeve teeth constantly rub together. That is the wear mechanism
that eventually causes the demise of the coupling. It is necessary to
change materials of construction to harden the couplings. The material
to be used must be compatible with induction hardening, carbonization,
or nitride hardening. When the tooth is hard it must retain its strength
to still carry the torque. Iron carbides and carbon nitrides provide
the surface hardness. While 1045 carbon steel is a popular standard
steel for gear couplings, Lovejoy also uses 4140 and other high alloy steels.

Lubrication
is certainly necessary to decrease the wear and reduce the friction
between the mating teeth. High speed couplings have oil lubrication. The
oil, which is circulated through filters and coolers, is sprayed into
the area of the teeth on one side and drained from the coupling on the
other side of the teeth. Grease would pose temperature rise problems,
might centrifuge out of the area needing lubrication, and would break
down requiring re-lubrication. Circulating oil has the advantage of
constant renewal.

F. Bigger Than Size 7

Large Gear Coupling

There
are several magic numbers when it comes to gear couplings. One is the
size cut-off between big and small. That number is arbitrarily set at 7,
but could be 9. The AGMA dimensional interchange goes to size nine for
gear couplings. Once the size rises to 7 and above, the number of
applications and therefore sales are very limited. A size seven gear
coupling has a bore capability of 9 or more inches (depends on key size
too) and a torque of approximately 1 million inch pounds. That torque
corresponds to 16,000 horsepower at 1,000 RPM. Not many applications go
that far and when they do the situation is special. Usually big gear
couplings are used on very low RPM and very high torque applications as
found in the steel and aluminum rolling mills, mine concentrators,
crushers, or rubber processors.

While
Lovejoy's gear coupling catalogs will show gear couplings up to size
30, Lovejoy's Downers Grove, IL and South Haven, MI facilities make and
stocks gear couplings up through size 15. Going above a size 15 (very
rare & very large couplings) requires Lovejoy's coordination with an
outside machine house and will add to a given product's lead time.

Very
large gear couplings (i.e. - above size 15) are often re-rated based on
improved materials, heat treating, and hardening. In reality the user
and designer are trading wear life for torque rating. The torque rating
can be used as a peak load or cyclic high and not always the normal
operating torque. The transmission shaft is loaded with torque only on
these applications so shaft capability is the same as or greater than
the coupling. We can offer that re-rate service, which in turn reduces
the coupling size, as long as the bore capability is not exceeded.